Sump for an anaesthetic vaporiser

ABSTRACT

A sump for an anaesthetic vaporiser comprises a vaporising chamber for liquid anaesthetic agent, a heater for liquid anaesthetic agent contained within the chamber, and a partition member of which at least a portion extends into the chamber to entrap a portion of liquid anaesthetic agent at a location in close proximity to the heater, the member having a partition outlet for anaesthetic agent vapor through the partition member at a position above the maximum level of anaesthetic agent within the partition member. The partition member allows a portion only of liquid anaesthetic agent within the vaporizing chamber to be heated, so that the temperature required of the liquid agent for operation of the vaporiser can be reached rapidly. The sump finds particular application in connection with the use of low boiling point anaesthetic agents such as 2 -(difluoromethoxy) 1,1,1,2 -tetrafluoroethane.

This invention relates to a sump for an anaesthetic agent vaporiser, andto a method of vaporising an anaesthetic agent. A known vaporiser forvolatile anaesthetic agents Includes a vaporising chamber for liquidanaesthetic agent, and an inlet into the vaporising chamber for acarrier gas which entrains anaesthetic agent vapor within the chamberabove the surface of the liquid anaesthetic agent and carries the vaporout of the vaporiser for adminstration to a patient. Such a vaporiser isdisclosed in GB-1224478.

The vapor pressure of many volatile anaesthetic agents is heavilydependent on the ambient temperature at which the vaporiser operates. Inorder to minimise the dependence of vapour pressure on the ambienttemperature in which the vaporiser operates, it is known to provide aheater within the vaporising chamber by which the temperature ofanaesthetic agent within the chamber can be controlled. It is to beunderstood however that the temperature of the anaesthetic agent ismaintained significantly below its boiling point; anaesthetic agentvapour is therefore carried out of the vaporising chamber not under itsown vapour pressure, but entrained in the carrier gas which is suppliedto the chamber.

A recently introduced anaesthetic agent,2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane has a relatively lowboiling point in the region of ambient temperature. Conventionaltechniques for administration of anaesthetic agents having boilingpoints well above ambient temperature, using vaporisers such as thatdisclosed in GB-1224478, are not generally applicable to this new agent.Recently developed vaporisers for low boiling point anaesthetic agentsare disclosed in EP-A-438218 and European patent application no.91306823.5. Subject matter disclosed In those documents is incorporatedin this specification by these references to the documents. A vaporiserincludes a vaporising chamber for liquid anaesthetic agent, in which theagent is vaporised. The vaporiser further includes a passage whichextends from an inlet for carrier gas to an outlet. Anaesthetic agentvapour from the vaporising chamber flows from the chamber under thepressure which is generated as a result of the liquid agent having beenheated to cause it to boil, to mix with carrier gas in the passage sothat the carrier gas which is discharged from the outlet, foradministration to a patient, is a mixture of carrier gas andanaesthetic: agent vapour.

SUMMARY OF THE INVENTION

In vaporisers for low boiling point vaporisers, it is particularlyimportant that the temperature of the liquid anaesthetic agent withinthe vaporising chamber be controlled carefully, because the pressure ofthe anaesthetic agent vapour is so heavily dependent on the temperatureof the liquid anaesthetic agent relative to its boiling point. It canalso be desired in many instances to reach the controlled temperature ofthe liquid anaesthetic agent as quickly as possible after replenishingthe supply of agent to the vaporising chamber or after first turning onthe vaporiser.

The present invention provides a sump for an anaesthetic vaporiser,which includes a partition member located in the vaporising chamber bywhich a portion of the liquid anaesthetic agent can be entrapped, forheating by means of an associated heater.

Accordingly, the invention provides a sump for an anaesthetic agentvaporiser, which comprises:

(a) a vaporising chamber for liquid anaesthetic agent,

(b) a heater for liquid anaesthetic agent container within the chamber,and

(c) a partition member in the form of a plate which extends across thevaporising chamber and cooperates with the internal wall of thevaporising chamber to entrap a portion of liquid anaesthetic agent at alocation immediately adjacent the heater, the plate having a partitionoutlet at a position above the maximum level of liquid anaesthetic agentwithin the space defined by the partition member and the internal wallof the vaporising chamber, through which anaesthetic agent vapour canpass through the plate.

The sump of the invention has the advantage that a quantity of liquidanaesthetic agent can be heated rapidly to a desired temperature, whichis above the boiling point of the anaesthetic agent, so that a pressureof anaesthetic agent vapour can be established to enable the vapour toflow from the vaporising chamber for administration to a patient. Theprovision of a partition member, by which a portion of the liquidanaesthetic agent in the vaporising chamber can be entrapped, allows thedesired temperature in the liquid anaesthetic agent to be reachedwithout having to heat all of the liquid in the vaporising chamber. Thesignificance of this advantage can be appreciated in the context ofrefilling of the vaporising chamber of a sump, when existing liquidanaesthetic agent which has been heated to cause vaporisation of theagent is supplemented by additional liquid agent which has not been soheated. The resulting reduction in the temperature of the liquid agentin the vaporising chamber will lead to an immediate reduction in thepressure of the anaesthetic agent vapour. This can have significantadverse consequences if prolonged and if the refilling takes place whileanaesthetic agent is being administered from the vaporiser to a patient.The sump of the present invention allows the pressure of the anaestheticagent vapour to be restored to a desired level quickly after such arefilling operation, so that any reduction in the supply of anaestheticagent to a patient under anaesthesia is minimised. It must further beappreciated that the likelihood of refilling of the vaporising chamberof a vaporiser, while agent is being administered to a patient, issomewhat greater in the case of2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane and other low boilingpoint anaesthetic agents than in the case of conventional agents becauseof the greater volumes of that agent which are administered duringanaesthesia.

In another aspect, the invention provides a method of vaporising ananaesthetic agent, comprising providing a sump for an anaesthetic agentvaporiser which comprises:

(a) a vaporising chamber for liquid anaesthetic agent,

(b) a heater for liquid anaesthetic agent contained within the chamber,and

(c) a partition member in the form of a plate which extends across thevaporising chamber and cooperates with the internal wall of thevaporising chamber to entrap a portion of liquid anaesthetic agent at alocation immediately adjacent the heater, the plate having a partitionoutlet at a position above the maximum level of liquid anaesthetic agentwithin the space defined by the partition member and the internal wallof the vaporising chamber, through which anaesthetic agent vapour canpass through the plate;

supplying heat to liquid anaesthetic agent entrapped within the saidspace by means of the heater to cause the anaesthetic agent to boil; andallowing anaesthetic agent vapour to flow from within the space definedby the partition member four supply to a patient, under the pressure ofthe anaesthetic agent vapour generated as a result of the heat suppliedto the liquid anaesthetic agent from the heater.

The sump of the invention may be arranged so that anaesthetic agentvapour is discharged through the partition outlet into the portion ofthe vaporising chamber outside the partition member, the vaporisingchamber including a chamber outlet for anaesthetic agent vapour from thesump. This arrangement has the advantage that carry over of liquidanaesthetic agent from the vaporising chamber with anaesthetic agentvapour can be minimised, by arranging for the flow of vapour from withinthe space defined by the partition member out of the sump to be along atortuous path, allowing liquid agent associated with anaesthetic agentvapour, for example as a result of agitation on boiling of the liquid,to drop back into the vaporising chamber. For example, the chamberoutlet and the partition outlet may be offset from one another so thatthe two outlets do not overlap with one another. The possibility ofminimising liquid carry over has the advantage that the quantity ofanaesthetic agent supplied to a patient can be controlled moreaccurately, since the agent which is being supplied is more likely to bein vapour form which can be quantified, and less likely to includeliquid which might vaporise between the vaporising chamber and thepatient, or within the patient's breathing circuit, increasingsignificantly the effective quantity of anaesthetic agent vapour whichis received by the patient.

When the partition member and the vaporising chamber each have outlets,the path which is followed by anaesthetic agent vapour passing throughthose outlets may be arranged to be tortuous by arranging the outlets sothat vapour flowing through each of the outlets follows respective pathswhich are non-parallel. For example, one of the outlets may be in avertical member, and the other of the outlets may be in a horizontalmember. In this event, it will be understood that the two outlets willbe such that they are offset from one another and do not overlap. Whenthe outlets are in members which are not perpendicular to one another,they are to be considered to be offset from one another and not tooverlap when the projection of one of the outlets onto the other outletdoes not overlap with that other outlet.

The partition member is as a plate which extends across the vaporisingchamber and cooperates with the internal wall of the vaporising chamberto define a space within the chamber for liquid anaesthetic agent to beheated by the heater. The plate may extend for example across thevaporising chamber or around a corner thereof. The plate includes atleast one opening in it located above the maximum level of the liquidanaesthetic agent in the vaporising chamber, through which anaestheticvapour can pass from within the space defined by the partition member,and it may include one or more openings located below the level of theliquid agent in the vaporising chamber, through which liquid agent canpass from outside the partition member, to replace that which has beenlost by vaporisation, and so to maintain the levels of liquid agentinside and outside the partition member approximately equal.

It can be preferred particularly that the heater has the form of anelongate member.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, which is a schematicrepresentation of an embodiment of sump for an anaesthetic vaporiser.

DESCRIPTION OF THE PREFERRED

Referring to the drawing, which shows a sump for an anaestheticvaporiser which includes a vaporising chamber 21, having an inlet 23 forliquid anaesthetic agent and an outlet 25 for anaesthetic vapour. Acylindrical heater 27 is located within a cylindrical finger 29 forheating liquid anaesthetic agent contained within the vaporising chamberto a temperature which is greater than the boiling point of the agent.

A plate 31 is provided between the heater and its associated finger andthe outlet 25 from the vaporising chamber. As shown, the plate extendsacross the chamber and cooperates with the internal wall of thevaporising chamber to define a space within the chamber for liquidanaesthetic agent which is to be heated by the heater. The plate hasopenings 33 extending through it at its upper end to allow the passageof anaesthetic vapour from within it towards the outlet 25 from thevaporising chamber. The plate also has openings 35 extending through itat its lower end through which liquid anaesthetic agent can pass intothe space of the vaporising chamber defined by the plate and theinternal wall of the chamber to replace the agent which is lost fromthat space by vaporisation.

The plate controls movement of the liquid anaesthetic agent within it asa result of the application to it of heat which can cause it to becomeagitated as it boils, so as to minimise the carriage of liquid agentinto the outlet 25 from the chamber together with anaesthetic vapour.

We claim:
 1. A sump for an anaesthetic agent vaporiser, which comprises:(a) a vaporising chamber for liquid anaesthetic agent, (b) a heater for liquid anaesthetic agent contained within the chamber, and (c) a partition member in the form of a plate which extends across the vaporising chamber and cooperates with the internal wall of the vaporising chamber to entrap a portion of liquid anaesthetic agent at a location in close proximity to the heater, the plate having a partition outlet at a position above the maximum level of liquid anaesthetic agent within the space defined by the partition member and the internal wall of the vaporising chamber, through which anaesthetic agent vapour can pass through the plate.
 2. A sump as claimed in claim 1, in which the partition outlet discharges the anaesthetic agent vapour into the vaporising chamber, and in which the vaporising chamber includes a chamber outlet for anaesthetic agent vapour.
 3. A sump as claimed in claim 2, in which the chamber outlet and the partition outlet are offset from one another so that the two outlets do not overlap with one another and so that the path followed by anaesthetic agent vapour leaving the sump through the partition and chamber outlets is tortuous.
 4. A sump as claimed in claim 2, in which the heater has the form of an elongate member.
 5. A method of vaporising an anaesthetic agent, comprising providing a sump for an anaesthetic agent vaporiser which comprises:(a) a vaporising chamber for liquid anaesthetic agent, (b) a heater for liquid anaesthetic agent contained within the chamber, and (c) a partition member in the form of a plate which extends across the vaporising chamber and cooperates with the internal wall of the vaporising chamber to entrap a portion of liquid anaesthetic agent at a location in close proximity to the heater, the plate having a partition outlet at a position above the maximum level of liquid anaesthetic agent within the space defined by the partition member and the internal wall of the vaporising chamber, through which anaesthetic agent vapour can pass through the plate; supplying heat to liquid anaesthetic agent entrapped within the said space by means of the heater to cause the anaesthetic agent to boil; and allowing anaesthetic agent vapour to flow from within the space defined by the partition member for supply to a patient, under the pressure of the anaesthetic agent vapour generated as a result of the heat supplied to the liquid anaesthetic agent from the heater. 